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CREATE SEQUENCE

CREATE SEQUENCE — define a new sequence generator

Synopsis

CREATE [ TEMPORARY | TEMP ] SEQUENCE [ IF NOT EXISTS ]name    [ ASdata_type ]    [ INCREMENT [ BY ]increment ]    [ MINVALUEminvalue | NO MINVALUE ] [ MAXVALUEmaxvalue | NO MAXVALUE ]    [ START [ WITH ]start ] [ CACHEcache ] [ [ NO ] CYCLE ]    [ OWNED BY {table_name.column_name | NONE } ]

Description

CREATE SEQUENCE creates a new sequence number generator. This involves creating and initializing a new special single-row table with the namename. The generator will be owned by the user issuing the command.

If a schema name is given then the sequence is created in the specified schema. Otherwise it is created in the current schema. Temporary sequences exist in a special schema, so a schema name cannot be given when creating a temporary sequence. The sequence name must be distinct from the name of any other sequence, table, index, view, or foreign table in the same schema.

After a sequence is created, you use the functionsnextval,currval, andsetval to operate on the sequence. These functions are documented inSection 9.16.

Although you cannot update a sequence directly, you can use a query like:

SELECT * FROMname;

to examine the parameters and current state of a sequence. In particular, thelast_value field of the sequence shows the last value allocated by any session. (Of course, this value might be obsolete by the time it's printed, if other sessions are actively doingnextval calls.)

Parameters

Notes

UseDROP SEQUENCE to remove a sequence.

Sequences are based onbigint arithmetic, so the range cannot exceed the range of an eight-byte integer (-9223372036854775808 to 9223372036854775807).

Becausenextval andsetval calls are never rolled back, sequence objects cannot be used if“gapless” assignment of sequence numbers is needed. It is possible to build gapless assignment by using exclusive locking of a table containing a counter; but this solution is much more expensive than sequence objects, especially if many transactions need sequence numbers concurrently.

Unexpected results might be obtained if acache setting greater than one is used for a sequence object that will be used concurrently by multiple sessions. Each session will allocate and cache successive sequence values during one access to the sequence object and increase the sequence object'slast_value accordingly. Then, the nextcache-1 uses ofnextval within that session simply return the preallocated values without touching the sequence object. So, any numbers allocated but not used within a session will be lost when that session ends, resulting in“holes” in the sequence.

Furthermore, although multiple sessions are guaranteed to allocate distinct sequence values, the values might be generated out of sequence when all the sessions are considered. For example, with acache setting of 10, session A might reserve values 1..10 and returnnextval=1, then session B might reserve values 11..20 and returnnextval=11 before session A has generatednextval=2. Thus, with acache setting of one it is safe to assume thatnextval values are generated sequentially; with acache setting greater than one you should only assume that thenextval values are all distinct, not that they are generated purely sequentially. Also,last_value will reflect the latest value reserved by any session, whether or not it has yet been returned bynextval.

Another consideration is that asetval executed on such a sequence will not be noticed by other sessions until they have used up any preallocated values they have cached.

Examples

Create an ascending sequence calledserial, starting at 101:

CREATE SEQUENCE serial START 101;

Select the next number from this sequence:

SELECT nextval('serial'); nextval---------     101

Select the next number from this sequence:

SELECT nextval('serial'); nextval---------     102

Use this sequence in anINSERT command:

INSERT INTO distributors VALUES (nextval('serial'), 'nothing');

Update the sequence value after aCOPY FROM:

BEGIN;COPY distributors FROM 'input_file';SELECT setval('serial', max(id)) FROM distributors;END;

Compatibility

CREATE SEQUENCE conforms to theSQL standard, with the following exceptions:

See Also